Level Editor improvements (Patch Notes 1.3.0)

Hi guys, I've been working on some other projects since the release of Cup of Ethanol, but I finally got back to improving the level select screen.

This is also a pre-update to prepare for potential workshop integration. I will give workshop integration a go, but if it turns into weeks of work I can't afford to do it.
So hopefully it's not too difficult to get working 🤞🤞🤞

Custom Levels

• Custom levels can be named and renamed (32 character limit)
• Custom levels can be deleted

Custom level select screen

• Custom level names are now shown instead of just a number (Note: You will have to rename any existing levels or it will still show the number)
• Empty level folders are not shown in the level list

Minor changes

Shows a specific error message when invalid enum opengl error occurs (since this happens often sadly 😢)